1. Field of the Invention
The present invention relates to an aluminum substrate for an electrophotographic photoconductor, where a surface thereof is covered with an anodized aluminum film. Also, the present invention relates to an electrophotographic photoconductor using the aluminum substrate.
2. Description of the Related Art
Heretofore, technical advances in electrophotography have been made in the filed of copier machines and recently have been applied in the field of laser printers and so on. The laser printers provide excellent image qualities and allow high speed and quiet printing operations in comparison with those of the conventional impact printers. Thus, most of the present recording devices, such as printers and copiers, adopt the electrophotographic technologies. Each of electrophotographic photoconductors (hereinafter, also simply referred as a photoconductor) to be provided in those recording devices is prepared by forming a photoconductive layer on a conductive substrate. In most photoconductors, each of them has a photoconductive layer consisting of organic materials and thus the photoconductor is designated as an organic photoconductor. Furthermore, it is now common practice to make each of the photoconductors as a structure having functionally separated layers (i.e., the photoconductive layer is divided into two different layers), where an under-coating layer, a charge-generation layer, and a charge-transport layer are stacked on a substrate in that order. The under-coating layer can be prepared by one of two different processes. In the first process, a resin-based material typically of polyamide or melamine is applied on the surface of the substrate. In the second process, on the other hand, an anodized film (hereinafter, simply referred as a film) is formed on the surface of an aluminum substrate by means of anodic oxidation. Generally, the second process is advantageous in terms of reliability under a high-temperature and high-humidity environment.
Typically, an organic photoconductor, where an organic material is used as its photoconductive layer's material, is formed by a wet-coating method that includes the step of immersing a substrate in a coating-liquid bath containing the organic material being dissolved or dispersed in a solvent. The required level of quality for the photoconductor is that the coated film should be made uniform (i.e., no roughness or irregularities) with no defect of any kind. Thus, the uniformity of the coated film largely depends on a surface condition (i.e., uniformity) of the substrate, remarkably in the case of using wet-coating.
When a substrate having a film formed on its surface is used, quality of the photoconductor itself is almost determined by a surface condition of the substrate after a sealing treatment following an anodic oxidation treatment. The surface condition here means uniform wettability of the surface, so that the coated film should have uniform wettability over the entire surface thereof. It has been clarified that a thickness of the photosensitive layer (particularly, a thickness of the charge-generation layer) becomes uneven when the wettability is not uniform, resulting in defects such as "uneven density" in print quality evaluation.
If contaminants such as oxides and ions are remained on the surface of the substrate before the step of coating the photosensitive layer, they tend to cause image defects such as "black spot" and "fog". Thus, the contaminants are generally removed by washing the substrate with alkali or acid. However, the washing process does not remove the contaminants to a sufficient degree when the sealing state of the film is insufficient, so that it often results in "black spot" or "fog". For determining whether the sealing state is sufficient, a criterion is an admittance value (Y.sub.20). According to the present invention, it is found that the value (Y.sub.20) is desirably less than 70 .mu.S. To decrease the admittance value (Y.sub.20), sealing treatment at a higher temperature for a longer time is required. Therefore, a value of less than 70 .mu.S can be obtained by the treatment at least at 80.degree. C. for 10 minutes.
In this description, the admittance value (Y.sub.20) is provided as a converted value of 20 .mu.m film thickness according to "Test methods for sealing quality of anodic oxide coatings on aluminum and aluminum alloys", JIS(Japanese Industrial Standard) H8683 (1994) by Japanese Industrial Standards Committee.
As a result of various investigations on any factors involved in the surface state of the film after sealing treatment next to anodic oxidation, changes in surface fine structure substantially affect the wettability. In general, surface configuration immediately after anodic oxidation treatment has a hexagonal columnar fine cell structure with fine pits of about 100 .ANG. in diameter present at the central part. A treatment for sealing these pits is referred to a sealing treatment, in which the film is hydrated in boiling water or steam in order to swell the film to seal the pits, or in general using a nickel acetate solution, the pits are sealed by a combination of hydration reaction of the film and filling with nickel hydroxide produced by hydrolysis of nickel acetate.
However, it has been found that in any of the above treatments, growth of the film by the hydration reaction abnormally occurs reticulately both in the horizontal direction and the vertical direction (film thickness direction), resulting in an irregular surface, which particularly affects the wettability of the photosensitive layer in a dip coating method, and the influence is particularly considerable when the film is treated at high temperatures. Furthermore, sealing treatment of the reticulated surface is not uniform over the entire surface and thus tends to generate irregularities.
Then, an object of the present invention is to provide a substrate for an electrophotographic photoconductor which realizes an admittance value (Y.sub.20) of 70 .mu.S or less, has suppressed growth of the film in a vertical direction, and has a uniform and smooth surface of high wettability with a high sealing degree, and an electrophotographic photoconductor using the substrate.
The inventors have conducted intensive studies for solving the above prior art problems and found that by adding a specific surfactant and the like to the prior art sealing agent, growth of film in vertical direction is suppressed and a uniform and smooth surface of good wettability with a high sealing degree can be obtained, thus accomplishing the present invention.